To clarify the acute effects of NO2 on gaseous exchange in the lung of mice, O2 and CO2 concentrations in respiratory gas, respiratory rate (RR), arterial blood pH, PaCO2, PaO2, lung wet weight and lung water content were examined using the head-enclosed method. The results of the present study indicate that in mice exposed to 5 ppm NO2 for 24 h the gaseous exchange in the lung and metabolic rate of O2 and CO2 in the body are accelerated, whereas in mice exposed to 10 ppm and 20 ppm NO2 the gaseous exchange in the lung is inhibited to change the gaseous metabolism.